Reference is made to commonly owned, co-pending U.S. patent application Ser. No. 12/793,398, filed Jun. 3, 2010, Ser. No. 12/793,441, filed Jun. 3, 2010, and Ser. No. 12/793,494, filed Jun. 3, 2010.
The present disclosure relates to a compact high intensity discharge lamp and especially to an arc tube for a compact high intensity discharge lamp, and more specifically to an arc tube of a compact metal halide lamp made of translucent, transparent or substantially transparent quartz glass, hard glass, or ceramic arc tube materials. It finds particular application, for example in the automotive lighting field, although it will be appreciated that selected aspects may find application in related discharge lamp environments for general lighting encountering the same issues with regard to salt pool location and maximizing luminous flux emitted from the lamp assembly. For purposes of the present disclosure, a “discharge chamber” refers to that part of a discharge lamp where the arc discharge is running, while the term “arc tube” represents that minimal structural assembly of the discharge lamp that is required to generate light by exciting an electric arc discharge in the discharge chamber. An arc tube also contains the pinch seals with the molybdenum foils and outer leads or lead wires (in the case of quartz arc tubes) or the ceramic protruded end plugs or ceramic legs with the seal glass seal portions and outer leads (in case of ceramic arc tubes) which ensure vacuum tightness of the “discharge chamber” plus the possibility to electrically connect the electrodes in the discharge chamber to the outside driving electrical components via the outer leads pointing out of the seal portions of the arc tube assembly.
High intensity discharge lamps produce light by ionizing a fill, such as a mixture of metal halides, mercury or its replacing buffer alternatives, and an inert gas such as neon, argon, krypton or xenon or a mixture of thereof with an arc passing between two electrodes that extend in most cases at the opposite ends into a discharge chamber and energize the fill in the discharge chamber. The electrodes and the fill are sealed within the translucent, transparent or substantially transparent discharge chamber which maintains a desired pressure of the energized fill and allows the emitted light to pass through. The fill (also known as a “dose”) emits visible electromagnetic radiation (that is, light) with a desired spectral power density distribution (spectrum) in response to being vaporized and excited by the arc. For example, rare earth metal halides provide spectral power density distributions that offer a broad choice of high quality spectral properties, including a wide range of color temperatures, excellent color rendering, and high luminous efficacy.
In current high intensity metal halide discharge lamps, a molten metal halide salt pool of overdosed quantity typically resides in a central bottom location or portion of a generally ellipsoidal or tubular discharge chamber, when the discharge chamber is disposed in a horizontal orientation during operation. Since location of the molten salt pool is always at the coldest part of the discharge chamber, this location or spot is often referred to as a “cold spot” location of the discharge chamber. The overdosed molten metal halide salt pool that is in thermal equilibrium with its saturated vapor developed above the liquid dose pool within the discharge chamber, and is located inside the discharge chamber of the lamp at the cold spot area, usually forms a thin liquid film layer on a significant portion of an inner surface of the discharge chamber wall. In this position, the dose pool distorts a spatial intensity distribution of the lamp by increasing light absorption and light scattering in directions where the dose pool is located within the discharge chamber. Moreover, the dose pool alters the color hue of light that passes through the thin liquid film of the dose pool.
Optical designers must address these issues when designing optics around high intensity arc discharge lamps that employ the described arc tube and discharge chamber arrangement. That is, configuration of the optical system must address absorbed, scattered and discolored light rays and the distorted spatial light intensity distribution caused by the distortion effect of the liquid halide dose pool in the discharge chamber. For example, in the past and even in contemporary automotive headlamp constructions, distorted light rays were/are either blocked out, by non light-transparent metal shields, or these light rays were/are distributed in directions that are not critical for the application. In other words, distorted light rays passing through the liquid dose film at the cold spot area of the discharge chamber are generally ignored. As such, this portion of emitted light from the arc discharge represent losses in the optical system since these distorted rays did/do not take part in forming the main beam of the beam forming optical system.
In an automotive headlamp application, for example, the distorted rays are used for slightly illuminating the road immediately preceding the automotive vehicle, or the distorted light rays are directed to road signs well above the road. Due to these losses, efficiency of the headlamp optical systems is typically no higher than approximately 40% to 50%. Optical losses due to beam distortions caused by dose pool in the discharge chamber in lighting systems for other applications may depend on the required beam characteristics, illumination and beam homogeneity levels, and other parameters.
As compact discharge lamps become smaller in wattage and additionally adopt reduced geometrical dimensions, a solution is required with the light source in order to avoid such losses in the optical assembly or system. An improved optical system equipped with discharge lamps of improved beam characteristics would desirably achieve higher illumination levels along with lower energy consumption of the overall lighting system.
Thus, a need exists to address the issues associated with the liquid dose pool located at the cold spot area within the discharge chamber of compact high intensity discharge lamps, and impact of this on performance and efficiency of optical systems designed around these lamps as a result of the uneven and distorted spatial and colorimetric light intensity distribution emitted by these lamps.